1. Field of the Invention
The present invention relates to an apparatus for detecting the movement of a heat source and particularly to such an apparatus utilizing a pyroelectric type infrared detector.
2. Description of the Related Art
When a certain dielectric is heated, the surface thereof produces a voltage or increases the electric charge thereon, so that the thermal energy is converted into an electric energy. Such a phenomenon is generally called "pyroelectric effect". Materials providing the pyroelectric effect are called "pyroelectric materials".
Many of such pyroelectric materials are ceramics. Pyroelectric type infrared sensors made from such pyroelectric materials have been used in systems for detecting the movement of a heat source. The heat source to be detected by the pyroelectric type infrared sensors is normally a person's body. The pyroelectric type infrared sensors are utilized in various applications such as an air conditioner capable of changing its wind direction depending on the movement of the person's body or a lighting equipment capable of changing its orientation depending on the movement of the person's body.
In order to detect the movement of the person's body, a heat source movement detecting system which utilizes a plurality of such pyroelectric type infrared sensors and which is constructed in accordance with the prior art has such a mechanism as is shown in FIG. 1.
Referring to FIG. 1, the system comprises a pyroelectric type infrared sensor 10a having an area to be observed which will be called "area A", a pyroelectric type infrared sensor 10b having an area B to be observed and pyroelectric type infrared sensor 10c having an area C to be observed. If each of these pyroelectric type infrared sensors, for example, the sensor 10a detects (+1), it means that a person has entered the area A. If the pyroelectric type infrared sensor 10a detects (-1), it represents that the person has exited the area A.
Although the pyroelectric type infrared sensor 10a can satisfactorily sense the entrance and exit of the heat source into and from the area A, it cannot sense the movement of the heat source within the area A except when any change in temperature occurs due to the movement of the heat source.
In order to avoid such a limitation, the other areas B and C are provided in the prior art in addition to the area A, as shown in FIG. 1. If the heat source moves from the area A to the area B, the temperature in the area A falls while the temperature in the area B rises. The fall and rise of temperature can be sensed by the pyroelectric type infrared sensors 10a and 10b. Thus, the system will judge that the heat source moved from the area A to the area B. Similarly, if the fall of temperature in the area B is simultaneously sensed with the rise of temperature in the area C, the system will judge that the heat source moved from the area B to the area C. Furthermore, if the fall of temperature in the area C is simultaneously sensed with the rise of temperature in the area A, the system will judge that the heat source moved from the area C to the area A.
In such a manner, the system can sense the movement of the heat source within a room when the plurality of pyroelectric type infrared sensors located in the room at the respective areas detect the movement of the heat source between the areas.
As described hereinbefore, however, the prior art system cannot detect the movement of the heat source in each of the areas A, B and C unless any change in temperature is involved by the movement of the heat source. In order to increase the degree of accuracy in measurement, it is required that each of the areas is reduced and also the number of pyroelectric type infrared sensors is increased. This will increase the entire size of the system and also the manufacturing cost.